[CLI] add doc for max_pres_fac

Author: amin1377

doc/src/vpr/command_line_usage.rst

@@ -1165,7 +1165,9 @@ VPR uses a negotiated congestion algorithm (based on Pathfinder) to perform rout
 
 .. option:: --max_pres_fac <float>
 
-    Sets the maximum present overuse penalty factor.
+    Sets the maximum present overuse penalty factor that can ever result during routing. Should always be less than 1e25 or so to prevent overflow. 
+    Smaller values may help prevent circuitous routing in difficult routing problems, but may increase 
+    the number of routing iterations needed and hence runtime.
 
     **Default:** ``1000.0``